SV40 transformed mouse embryo cells were lavelled in culture with 35 S-methionine, protein was extracted and aliquots of the extract treated with hamster anti-T antigen serum, or with buffer, to produce SV40 T antigen, or control (N), immuno-precipitates (IP), respectively, which were analyzed by SDS-polyacrylamide gel electrophoresis. Three specific bands were found in T reactive solutions, of molecular weights about 94,000 and 19,000 and 53,000 daltons, for SV40 large T and small t antigens, and the cellular 53K protein, respectively. A similar procedure is being used to attempt to detect RNA in these IP proteins, after incubating the cultures with 3H-uridine, an RNA precursor. The IP from these cells possessed low levels of radioactivity, which was not found onthe gels. Work is continuing to determine the nature of the radioactivity associated with the IP, which seems to be absent from the gels used to analyze them.